This invention relates to a method of fabricating a multitublar heat exchanger and more particularly, a method of jointing heat transmitting tubes made of titanium with a header made of titanium.
Generally, a tube expanding method and a welding method are used as a method of jointing a heat transmitting tube with a header for fabricating a multitublar heat exchanger. The tube expanding method is illustrated in FIG. 1, in which the end portion of a heat transmitting tube 1 is inserted into a hole of a header 2 and then expanded so as to forcibly engage the expanded end portion of the tube 1 with the inner surface of the hole of the header 2. The welding method is illustrated in FIGS. 2 and 3, in which the heat transmitting tube 1 is welded to the header 2 by suitable welding means.
In a nuclear or steam electric power generating plant, there are installed a steam condenser, an air extractor, a gland-exhauster steam condenser or the like in which fluid having a low temperature and a low pressure is used. In such devices, for example a condenser having a large volume, although the heat transmitting tube is jointed with the header by the tube expanding method, the tube is subjected to a large force, which must be withstood by the jointed portion. Therefore, in the condenser used in a nuclear power plant, a plurality of grooves 5 are provided as shown in FIG. 4, for the hole 4 of the header 2 to firmly engage the header 2 with the heat transmitting tube 1 and to increase the grasping force of the header when the inserted tube end is expanded. However, in a case where a titanium tube is used as a heat transmitting tube 1, it is generally required for the titanium tube to have a thin thickness of about 0.5 mm in view of its working cost and its heat transfer rate. Moreover, titanium possesses a spring-back characteristic, so that it is impossible to obtain a desired grasping force between the titanium tube and the titanium or titanium clad header. For the reasons described above, the grapsing force is only one third of that of a heat exchanger using brass tubes. Therefore, in a heat exchanger using titanium tubes, when a pump for circulating cooling water is stopped and a large pressure is applied, the titanium tubes are subjected to an axial force, which mainly acts on and severely affects the joints between the tubes and the header.
In order to obviate the above defect, an improved method has been proposed wherein a titanium tube is welded to a header through a titanium plate or titanium clad plate. This method, however, also includes a problem that the titanium plate absorbes an active gas such as oxygen or nitrogen under a high temperature prevailing at the time of welding thereby weakening the same. To eliminate this problem, although it is desirable to carry out the welding in a shielding atmosphere of innert gas, since the condenser has generally a large volume, it is impossible to completely shield the portions of the titanium tube and the titanium header which are to be welded from the surrounding air. Furthermore, since the titanium tube is thinner than the titanium header, when welding them, the titanium tube will firstly be heated and melted before the titanium header is melted and the titanium tube 1 is partially melted away thereby to form a hole through the tube before completion of the welding.